I think every mom has to seek her own comfort level. We get enough guilt as it is, for tons of different things. If another mom doesn't want to let it all hang out, then she shouldn't be made to feel like she's being a Bad Lactivist for it if that is her preference, any more than a mom who feels okay with not covering should be made to feel bad about not covering. I've heard variants of this as a Bad Feminist argument lots of times: if you take your husband's name/stay home with your kids/shave your legs/wear makeup/whatever, you are undermining feminism. It just gets ridiculous, with people wasting their own and others' time judging others rather than working on their common cause.
I own a cover and have used it exactly once--in the privacy of my own home, to see if it helped with my daughter's distractibility. It didn't. I do think it's a little peculiar that in one of the two breastfeeding groups I sometimes go to, almost all of the moms nurse with covers, and this is in a room with only moms, babies, and (female) LCs. Oh well, different strokes.